Trypanosomiasis is caused by Trypanosomatidae and 200,000 to 300,000 cases of trypanosomiasis are said to occur annually. The number of patients with African sleeping sickness cannot be determined due to low reliability of surveillance data at present. According to the WHO, at least 150,000 people died of the sickness in 1996 and 100,000 people or more have its aftereffects. Moreover, a disease called as nagana does more serious damage to domestic animals, with several hundred thousands of cattle which are to be protein sources for people lost annually to the disease. Furthermore, Trypanosoma has made it impossible to do livestock farming in savanna with an area of about 10,000,000 km2 equal to that of the United States of America. As mentioned above, African sleeping sickness greatly has hampered health and economic development of African people. This is the reason why the WHO has described the sickness as one of infectious diseases to be controlled.
African sleeping sickness is a tsetse-fly-transmitted protozoal infection caused by Trypanosoma and the protozoan appears in the bloodstream in about 10 days after infection. At the initial phase of infection, the protozoan multiplies in the bloodstream and cause fever, malaise, headaches, aching muscles and joints, and itching to occur and progress. During the chromic phase of infection, the central nerve is affected to show symptoms such as mental confusion and generalized convulsion, and finally the infection causes lethargy, eventually leading to death.
Trypanosomiasis in domestic animals is an epidemic which is caused by the pathogens Trypanosoma brucei brucei, T. evansi, T. congolense, and T. vivax and affects domestic animals such as horses, cattle, pigs and dogs as well as mice, guinea pigs, and rabbits. Particularly cattle and horses are worst affected by the epidemic, which is almost fatal to cattle and horses, causing symptoms such as anemia, edema, and weakness and, in one month after infection, death.
Some pharmaceuticals such as pentamidine, melarsoprol, and eflornithine are used for treating trypanosomiasis and there was sentiment in the 1960s that it might possibly be eradicated. These pharmaceuticals are, however, old and have decreasing efficacy. Particularly, tolerance to melarsoprol as an arsenic-containing agent is a serious issue, and patients showing no effect of the agent are therefore in such a dire situation that those patients can do nothing but wait to die.
Trypanosoma primarily lives in the bloodstream of the human body. This energy metabolism in the blood stream form depends on the glycolytic pathway localized in the organelle, known as glycosome, characteristic of the protozoan while the so-called oxidative phosphorylation in mitochondria does not work. To efficiently drive this glycolytic pathway, the NADH generated has to be reoxidized. In this reoxidation, the glycerol-3-phosphate oxidation pathway in mitochondria plays an important role. The terminal oxidizing enzyme in this oxidation pathway functions as a quinol oxidase having a reduced ubiquinone as an electron donor, and has properties significantly different from those of cytochrome oxidase in an aerobic respiratory system present in the host. It is particularly noted that the terminal oxidase of the oxidation system is non-sensitive to the cyanide which rapidly inhibits the cytochrome oxidase in the host. Many researchers around Western countries have thus attempted to develop pharmaceuticals targeting this cyanide-insensitive oxidase only to fail to create effective ones having a highly selective toxicity.
Under these circumstances, the present inventors have discovered that isoprenoid based physiologically active substances, ascochlorin, ascofuranone and derivatives thereof, particularly ascofuranone specifically inhibits the glycerol-3-phosphate oxidation pathway in Trypanosoma at an extremely low concentration of the order of nM and filed a patent application (Japanese Patent Application Laid-Open Publication No. H09-165332). That is, the inhibitory effect of ascochlorin, ascofuranone, and derivatives thereof on the glycerol-3-phosphate-dependent respiration was examined in the mitochondrial specimen prepared by mechanically homogenizing the bodies of T. brucei brucei multiplied in the rat bloodstream with glass beads followed by differential centrifugation. Antimycin A3, myxothiazol, and stigmatellin, all of which are known as Q-cycle inhibitors, had absolute amounts of 50% inhibition of 48,600, 21,500, and 18,600 pmol/mg protein respectively, whereas ascofuranone showed the inhibitory effect at a concentration as extremely low as 25 pmol/mg protein.
With the aim of making ascofuranone in practical use, it was found that there was a need to discover a pharmaceutical which replace glycerol and exhibit an effect of the combined use in a small amount, and that use of an alkaloid compound having an indole skeleton belonging to the plant family Simaroubaceae with ascofuranone provided life-lengthening or curing effect in African sleeping sickness, and then a patent application was filed (Japanese Patent Application No. 2003-24643 (Japanese Patent Application Laid-Open Publication No. 2004-231601)). Moreover, the present applicant devised a novel phenol derivative that can be used as an antitrypanosoma preventing/treating agent (WO2005/037760).
Meanwhile, Cryptosporidium parvum, which causes cryptosporidiosis, is the 5 μm smallest, oval-shaped intestinal protozoan. This intestinal protozoan belongs to Sporozoa Coccidia, parasitizes inside the microvilli of the mucosal epithelial cells, and multiplies while repeating in turns asexual reproduction and sexual reproduction. Many of the oocysts formed in sexual reproduction are excreted in the faeces, which become sources of infection to other individuals. The pathological condition of cryptosporidiosis is as follows: the main symptom is severe watery diarrhea associated with abdominal pain from 4-5 days to about a week, and about half of the cases have vomiting and mild fever, and most cases have no rectal bleeding. For cryptosporidiosis patients with normal immune function, the diarrhea subsides in a week or two even without any treatment, while for immunodeficient cryptosporidiosis patients, who has poor antibody formation, the diarrhea is intractable, can become chronic and more severe as there is no effective therapy for such diarrhea.
It is said that the detection rate of Cryptosporidium oocysts from diarrheal stool is about 10% in Africa and Central and South America, 5% in the Asia-Pacific regional, 3.5% in Europe, and 1.6% in North America, and that hundreds of millions of people worldwide are infected with Cryptosporidium annually.
Cryptosporidium is a zoonotic pathogen that infects small intestinal epithelial cells and causes severe diarrhea. This is of interest as a pathogen causing emerging infectious diseases that cause mass water-borne infection, as well as pathogen causing endemic tropical diseases, or opportunistic pathogen in immunodeficient patients.
Cryptosporidium, which is a minute protozoan discovered in 1907, has not attracted medical attention due to its unclear pathogenicity. In 1976, this protozoan was reported to cause diarrhea to human, and in 1982 C. parvum was detected by the U.S. Centers for Disease Control and Prevention (CDC) from many AIDS patients complaining of severe diarrhea and abdominal pain. Since then, Cryptosporidium has attracted much attention.
If AIDS patients are infected with C. parvum, then repeated multiplication of C. parvum takes place in the intestinal mucosa of the host over one year and the outcome is debilitation and eventually death in many cases even though symptomatic therapy is given. If healthy individuals are infected by C. parvum, cryptosporidiosis is characterized by watery stool to mucous stool and loose stool with abdominal pain as its chief symptom although it lingers and becomes severe in some cases in infants and the elderly.
Cryptosporidium is widely prevalent among animals, for example, domestic animals such as cattle, pigs, and sheep as well as dogs, cats, and mice. Particularly, for calves, individuals less than one month old are likely to be infected by Cryptosporidium to develop cryptosporidiosis. The symptoms such as watery diarrhea along with abdominal pain, and fever persist for three days to about a week. Additionally, the symptoms are exacerbated by mixed infection with viruses, bacteria, and Coccidia.
Reports on cryptosporidiosis in animals have been made from researchers from different countries in the world and cattle worldwide were found to be contaminated by the present infection. Also, in North America, detailed data are available, indicating that 15 to 60% of calves in the United States of America and Canada are infected. In 1997, a nationalwide survey was also conducted in Japan, and the survey found that protozoans were observed in 2.14% of faeces for cattle, and 1.10% of faeces for pigs.
As described above, Cryptosporidium is parasitic in not only humans, but also in animals over a wide range, leading to considerable damage particularly to domestic animals.
Any therapeutic drug that is clearly effective against cryptosporidiosis has not been found yet. Immunologically normal people can be cured of their Cryptosporidium infection with their self-immunocompetence, whereas patients with an immunologic deficiency such as AIDS patients fail to be cured of their Cryptosporidium infection with their self-immunocompetence, and diarrhea caused by the present infection may be sometimes fatal. Therefore, there is a need for Cryptosporidium-cides or protozoacides.
At present, multidrug therapy combining antibacterial drugs such as Lactobacillus preparations, azithromycin (Zithromac®), clarithromycin (Clarith® and Klaricid®), and roxithromycin (Rulid®), has been used. The therapy has not been shown to be highly effective although some reports say that the therapy improves the diarrhea (“®” refers to registered trademark).
Under these circumstances, the present applicant has earlier proposed phenol derivatives useful as preventing or treating cryptosporidiosis preventing/treating agent (Japanese Patent Application Laid-Open Publication No. 2005-112755 (Japanese Patent No. 4553569)).
However, excellent compounds have not been virtually found that are effective at lower concentrations, can be synthesized more easily, and are safer than ascofuranone for the purpose of prevention and treatment of diseases including Trypanosomiasis and cryptosporidiosis induced by protozoans.